This invention relates to a process for preparing a rhodium nitrate solution employed for producing a catalyst having the excellent stability and the like.
Conventionally, in order to prepare a rhodium nitrate solution, a caustic soda aqueous solution, aqueous ammonia or the like is added to a rhodium chloride solution to form rhodium hydroxide therein, and the resulting precipitate is dissolved in nitric acid to prepare rhodium nitrate.
However, the rhodium nitrate solution prepared according to the above process produces a colloidal precipitate to make itself turbid when the solution is heated. The solution also produces a similar colloidal precipitate even at room temperature when it is allowed to stand for one week or more. Although the reason the precipitate is formed is not necessarily apparent, the following ones are considered to be acceptable. That is, (i) part of the rhodium nitrate is converted into rhodium hydroxide through hydrolysis; (ii) since the rhodium hydroxide is not completely converted into the rhodium nitrate by simply dissolving the rhodium hydroxide in the nitric acid, the rhodium hydroxide which easily precipitates remains in the solution so that the hydroxide deposits with the elapse of time; and (iii) the conversion of the rhodium chloride to the rhodium hydroxide is insufficient so that the rhodium hydroxide contains impurities which promote the deposition of the rhodium nitrate. The said decomposition and the like lower such a characteristic of rhodium as catalyst efficiency.
Heating or concentration of a rhodium nitrate solution may be necessary depending on a purpose of use, and further the longer term storage may be required. The conventionally prepared rhodium nitrate solution can not keep its characteristic in such conditions.